1. Field of the Invention
The present invention relates generally to a colorimeter for color displays and more particularly to a high accuracy colorimeter capable of correcting measurement errors caused by deviations of the spectral responsivities of a light receiving unit from the color matching functions.
2. Description of the Prior Art
Tristimulus colorimeters, hereinafter referred to simply as colorimeters, are in wide practical use today as instruments capable of easily measuring the color of a light source. In the colorimeters, however, measurement errors caused by deviations of the spectral responsivities thereof from the color matching functions x(.lambda.), y(.lambda.), and z(.lambda.) are unavoidable. The larger the difference between the spectral distribution (chromaticity) of a light source used for calibration and that of a light source to be measured is, the larger the measurement errors become. Accordingly, if a colorimeter is calibrated by the use of a single reference light source such as, for example, an incandescent lamp, the kind of light sources to be measured to which measurements can be performed with sufficiently high accuracy is limited. In other words, the range of chromaticity in which the accuracy is sufficiently high is limited to a certain range in the proximity of the point of chromaticity of the reference light source.
To solve this problem, a plurality of reference light sources have hitherto been prepared for a single colorimeter and calibration has been performed upon selection of a reference light source of the type close to a light source to be measured. This method, however, is extremely inconvenient because it requires many reference light sources and the calibration prior to measurements. To improve this, a method is known in which correction is performed upon selection of an appropriate calibration factor from among a plurality of calibration factors in compliance with the kind or the measured value in chromaticity of a light source to be measured. In this case, the calibration is performed in advance by the use of a plurality of reference light sources and the calibration factors are stored in a colorimeter.
Even in this kind of correction method, not only the calibration by the use of the plurality of light sources is a considerably time-consuming work but also the number of the reference light sources for use in calibration is still limited. Accordingly, sufficient correction cannot be performed with respect to light sources having respective chromaticities apart from the points of chromaticity of the reference light sources.
Furthermore, a method of performing correction from the coordinate of a point of chromaticity measured is known but cannot sufficiently correct errors. The reason for this is that light sources having the same chromaticity are not always the same in spectral distribution and errors in chromaticity measurements differ according to the difference in spectral distribution.
Particularly, in the measurements of lights emitted from color displays such as CRTs, the chromaticity thereof greatly changes according to the mixing ratio of the three primary colors of red, green, and blue. Accordingly, in order to perform high accuracy measurements, it is necessary to prepare reference lights of the three primary colors or a white light obtained by mixing these lights at a given ratio so that calibration may be performed upon selection of an appropriate reference light according to the color of a light to be measured (color temperatures of respective colored lights in the case of the white light). Otherwise, a plurality of colorimeters are required for respective colored lights only. Even in applications where such reference lights or colorimeters are prepared, measurements cannot be performed with sufficient accuracy in the range of chromaticity positioned between the points of chromaticity of the reference lights.
In addition, there are many colorimeters having a function of simultaneously measuring not only the chromaticity or the color temperature of a light to be measured but the photometric quantity such, for example, as the illuminance or luminance by making use of outputs from a light receiving unit close to y(.lambda.) of the color matching functions i.e., the spectral luminous efficiency of the standard photometric observer. The spectral responsivities of the light receiving unit close to y(.lambda.), however, always include deviations from the spectral luminous efficiency. Accordingly, if a light to be measured differ in spectral distribution from a light source used for calibration, there arise heterochromatic photometry errors. In particular, when the photometric quantity of colored lights emitted from a color display or the like is measured by a colorimeter calibrated by the use of a white light emitted from, for example, a reference incandescent lamp, measurement errors become considerably large. Up to this time, however, there are no colorimeters capable of automatically correcting such heterochromatic photometry errors.